1. Field of the Invention
This invention relates generally to distortion cancellation techniques and more specifically to feed-forward networks and techniques for altering the amount of signal noise and distortion provided by amplifying devices.
2. Description of the Prior Art
Amplifiers are customarily used for enhancing electrical signals. However, a common problem associated with such signal enhancement is the addition of amplifier noise and distortion to the electrical signal. This is especially true when power amplifiers are utilized. Traditionally, feed-forward networks and techniques have been used to reduce the amount of signal noise and distortion generated by such amplifying devices.
FIG. 1 is a functional diagram of a traditional feed-forward system 1 coupled to an amplifier 5 which generates noise and distortion when amplifying an input signal. More specifically, the amplifier 5 has an input port at 10 on which is provided an undistorted input signal and an output port at 15. The feed-forward network includes a directional coupler 20 which is operatively connected to the amplifier input port in order to provide an undistorted input signal. A directional coupler 25 is operatively connected to the amplifier output port in order to provide a sample output signal of the amplifier. The sample output signal contains an input signal component, whose amplitude is altered by the amplifier, as well as an unwanted noise and distortion component added to the amplified input signal by the amplifier.
The conventional feed-forward network shown in FIG. 1 includes a manually adjustable phase adjustment means 30 and a manually adjustable amplitude adjustment means 35. The phase and amplitude adjustment means are connected in series with each other and they are coupled to the output of the directional coupler 25. The amplitude of the sample output signal is adjusted by the amplitude adjustment means 35 such that the input signal component of the sample output signal has the same amplitude as the input signal. The phase adjustment means 30 alters the phase of the sample output signal so that the sample output signal is 180.degree. out of phase with the input signal.
The phase and amplitude adjusted sample output signal and the undistorted input signal are provided to a coupling device 40 in order to remove or cancel the input signal component from the amplitude and phase adjusted sample output signal. After the signals are combined, the coupling device will provide a signal which essentially consists of only the noise and distortion components added to the input signal during amplification by the amplifier 5.
The output signal from coupling device 40 is provided to an error amplifier 45 which can amplify the coupling device output signal to substantially match the amplitude of the noise and distortion component of the amplifier output signal. The coupling device output signal is then further adjusted in amplitude and phase by fine amplitude and phase adjustment means 50, 55 so that the signal has substantially the same amplitude and is substantially 180.degree. out of phase with the undesirable noise and distortion component of the amplifier output signal. The amplitude and phase adjusted error amplifier output signal is then combined by means of directional coupler 60 with the amplifier output signal. As a result of the amplitude and phase adjusted error amplifier output signal being equal in amplitude and 180.degree. out of phase with the amplifier output signal, the unwanted noise and distortion that was added by the amplifier is canceled from the amplifier output signal. The remaining output signal is free of noise and distortion and it corresponds to an amplified, undistorted input signal.
In the conventional feed-forward system of FIG. 1, the amplitude and phase manipulation of the signals is typically produced by components that are manually adjusted. This usually results in poor noise and distortion cancellation performance as the characteristics of the input signal vary. Environmental effects and aging may affect component values and require that the selected phase and amplitude settings be changed. Since the noise and distortion components may not be completely eliminated from the amplifier's output signal because of improper manual settings, the performance of the system can be significantly degraded. As the input signal parameters such as signal frequency or amplitude change, tedious readjustments of the components are often required. Furthermore, the gain or phase of the amplifier may change when the frequency or amplitude of the input signal changes requiring additional adjustments of the system components.